The present invention relates to an active matrix liquid LCD (Liquid Crystal Display) and, more particularly to an LCD with an improved display characteristic.
An active matrix LCD of the type using a twisted nematic (TN) system is extensively used. This type of LCD includes a pair of transparent electrodes for driving a liquid crystal layer. The pair of electrodes are respectively arranged face-to-face on a pair of substrates. An electric field is applied to the liquid crystal layer in a direction substantially perpendicular to the surfaces of the substrates, thereby controlling the orientation of liquid crystal. In an active matrix drive system, pixels are arranged in a matrix in regions defined by a plurality of scanning wirings and signal wirings. A single switching device is assigned to each of the pixels. Signals are sequentially fed to the scanning wirings and signal wirings in order to operate the switching devices belonging to the pixels selected. The active matrix drive system realizes a high definition LCD having a great number of pixels.
However, the problem with the active matrix LCD is that because the liquid crystal molecules rotate in the direction substantially perpendicular to the substrates, transmissivity varies in accordance with the rotation angle of the molecules and therefore with the direction in which the LCD is seen. As a result, brightness noticeably changes with a change in the viewing direction and renders the display of halftone difficult while reducing view angle. Moreover, because twisted orientation exists in a plane parallel to the substrates, the molecules are subjected to restriction in the direction of twist when rotating in the vertical direction. The rotation therefore needs a substantial period of time to complete and reduces the response speed for display.
There is an increasing demand for an implementation for reducing dependency on view angle due to the increasing screen size of an LCD. Because the angle to a given visual point differs from one region to another region of the screen, display particularly differs from one edge to the other edge when a change in brightness or color ascribable to view angle is great. If dependency on view angle is small, then it is possible to display information evenly without regard to view angle, i.e., to allow two or more persons to recognize the information in the same condition. Today, there is an increasing demand for the display of a moving picture as distinguished from a still picture. A low response speed for display causes the previous image, i.e., to remain at the time of switching of display as a residual image. To display a moving image in an easily recognizable condition, it is necessary to increase the response speed in order to reduce the residual image.
In light of the above, an LCD capable of applying an electric field to a liquid crystal layer in a direction substantially parallel to substrates so as to control the orientation of liquid crystal is available, as disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 56-88179 and 6-273803. However, an LCD taught in Laid-Open Publication No. 56-88179 has a problem that when it is driven by the active matrix scheme, scanning wirings and signal wirings should be arranged in parallel to or perpendicularly to electrodes. As a result, the orientation of the liquid crystal is disturbed by the potentials of the wirings, rendering the display characteristic of the LCD defective. The LCD taught in Laid-Open Publication No. 6-273803 allows liquid crystal molecules to rotate in a plane parallel to substrates and thereby reduces the variation of transmissivity ascribable to view angle as far as possible, so that a desirable view angle characteristic is achievable. However, the problem with this LCD is that a heavy load acts on the liquid crystal molecules during rotation in the plan parallel to the substrates and thereby lowers the response speed.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Patent Laid-Open Publication Nos. 6-160878 and 7-56148 and Proceedings of 19th Liquid Crystal Forum, pp. 308-309, September 1993.